It is beneficial to monitor the strength of hypochlorite solutions used for disinfection purposes. Commercial hypochlorite degrades over time as a function of temperature and in the presence of certain impurities. It is also commonly diluted prior to dosing and it is beneficial to know the strength of the diluted solution. On-site electrolytic generated hypochlorite may also vary in strength and it is useful to know the concentration for the purposes of process optimization.
Many compounds absorb ultraviolet (UV) or visible (Vis.) light. The diagram 500 of FIG. 5 shows a beam of monochromatic radiation 510 of radiant power P0, directed at a sample solution 520. Absorption takes place and the beam of radiation leaving the sample 530 has a reduced radiant power P.
Transmittance may be calculated according to the following equations:Transmittance, T=P/P0; and% Transmittance, %T=100 T.
Absorbance may be calculated according to the following equations:Absorbance, A=log10 P0/P; A=log10 1/T; A=log10 100/% T; andA=2−log10% T. 
The relationship between absorbance and transmittance is illustrated in FIG. 6. If all the light passes through a solution without any absorption, then absorbance is zero, and percent transmittance is 100%. If all the light is absorbed, then percent transmittance is zero, and absorption is infinite.